Here are my thoughts regarding the 7D/70D AF vs 6D AF, having used and owned all 3 (at the same time).
Using center point, the 6D is just as much capable as the 7D/70D. AF accuracy in my experience is actually better on the 6D, and it does not seem to be affected by the infamous focus shift the 7D/70D produces under different lights (natural vs. incandescent --> quite the focus shift when using a fast lens.)
Even one-shot focus speed is very similar, I made a video, where I thoroughly compared my 7D and 6D under medium lights in one-shot mode, using 2 different lenses (Canon 24-105L, and Tamron 24-70 2.8 VC). Both center and off center points were just as fast, often slightly faster on the 6D. I didn't want to believe my eyes. I recorded both cameras and made a split-screen video in Premiere Pro where I synced the start (shutter button half press), and timed the focus confirmation beep. The 6D was significantly faster with the Tamron lens than the 7D with the Tamron lens (both center and peripheral points), but that may be due to some minor firmware "issue" that slows down the Tamron on the 7D (the 70D is similar with the Tamron). Even with the 24-105L on both cameras, the 6D registered more wins in this contests (the margin of wins were only a few milliseconds though, nothing perceptible in real life.)
Tracking is pretty good with the 6D too with the center point. A couple weeks ago I shot a charity basketball game from the stands using my Sigma 70-200 2.8 OS, and I was satisfied with the results.
So far it looks good for the 6D...
Now, when it comes to the outer points, the story changes a bit. Even though the general consensus is that the 6D's AF system is an improvement over the 5D2's (both center and off-center points) the fact is you still only have non-cross type outer points. So you gotta be careful and keep in mind the orientation of the non-cross type points. Under most conditions, if you have a good target, you can use the outer points with confidence. It's physics; if they see a good target, e.g. someone's eye, they lock on and they are accurate. But when conditions go downhill, or occasionally they encounter a pattern that they can't pick up due to orientation, there may be problems. Obviously, you don't have to worry about this with the 7D/70D (and the 5DIII). On these bodies both one-shot and AI servo is good with these outer points; so we could say that the range of conditions that these outer points track well is much wider than with the 6D. It's not like the 6D won't track with the outer points but when conditions get to a point, there will be more keepers with the 7D/70D/5DIII, probably the most keepers with the latter. So it largely depends on the task and the conditions how much better the more advanced AF system will do. But anyone who buys a 6D is not buying it for off-center AF point tracking purposes during twilight
So that's why it is very important to determine what you're going to use your body for. There are fairly simple tasks that a 6D can do with ease (a dog walking or jogging at steady speed and predictable direction is one of them in my opinion.) However, if the motion gets more complex or you need to track with the outer points for composition reasons under more challenging conditions, it's obviously better to trust a more advanced AF system with that task.
I really like my 6D, I think it's an excellent value and generally I'm not challenging its AF system with tasks that I don't expect it to perform well at. Occasionally, I run into situations where I need to track a bird in flight with the center point and it performs well (and birds are mainly what I shoot.) But I'd still like to afford a 5DIII some day.